The present invention relates to a recording/reproducing method and a recording/reproducing apparatus for magnetic tapes, the method and apparatus being capable of managing size changes of magnetic tapes.
Magnetic tapes are used in various applications such as audio tapes, video tapes and computer tapes. Particularly in the field of data backup tapes, magnetic tapes having recording capacities of several hundreds of gigabytes or more per reel have been commercialized along with increasing mass storage of hard disks, which are targeted for gigabyte backup. Further, mass-storage backup tapes which will go beyond 1 terabytes from this time forward have been proposed, for which implementation of higher recording densities is indispensable.
With a view to increasing the capacity of data that can be recorded on a magnetic tape, it has been in progress to increase the number of data tracks by reducing the width of a data track, which is so called recording density enhancement of data tracks. For example, with a magnetic tape having a width of ½ inch (about 12.7 mm), the number of data tracks reaches about several hundreds of tracks, causing the width of data tracks to be as narrow as 20 μm or less.
In parallel with this, the head for recording and reproducing data on a magnetic tape is provided by adopting a multichannel head in which a plurality of magnetic heads are disposed on a head unit. In the multichannel head, a tracking servo technology have been introduced so as to allow each magnetic head to accurately trace on each data track of narrow width. This is a technology that a servo signal previously written on the magnetic tape is read by a servo head provided in the head unit and then an actuator is driven according to the read signal so that even if the track position has changed widthwise during recording and reproducing operation, the head unit can be positionally controlled in the widthwise direction of the magnetic tape, thus enabling each magnetic head to follow each data track.
Such tracking servo technologies come in two kinds of methods. One of the technologies is the magnetic servo method, which includes forming a servo signal on a magnetic layer by magnetic recording and reading this signal magnetically to do servo tracking. The other is the optical servo method, in which a servo signal made from a recessed portion array is formed on a back layer by laser irradiation and then read optically to perform servo tracking.
A description is given below about the tracking servo method by taking the magnetic servo method as an example. FIG. 1 schematically shows a relative positional relation between a magnetic tape 1 and a head unit 2 in this case, and FIG. 2 schematically shows the structure of the head unit 2. As shown in these figures, a servo track group 4 in which servo signals have been written is provided on the magnetic layer of the magnetic tape 1. On the head unit 2, on the other hand, normally provided are at least one servo head 12a/12b and a plurality of data head groups 11 for higher data transfer rate. In recording and reproducing operations, the head unit 2 runs on the magnetic tape 1 in a longitudinal direction X. In this operation, if the magnetic tape 1 has swung in the widthwise direction (Y direction in FIG. 1), the signal derived from the servo head 12a/12b that traces the servo track changes and the actuator (not shown) is activated responsively, by which the whole head unit 2 is moved in the widthwise direction (Y direction) of the magnetic tape 1, with the result that the positional relation between the magnetic tape 1 and the head unit is recovered correctly.
One example of such tracking servo technologies is described in JP 2002-157722 A. In this case, the head unit is equipped with not only a normal servo track reading head but also a preceding reading head that reads positional information about the servo track, with a view to further improving the tracking servo precision.
In this connection, the magnetic tape slightly varies in the size of width along the longitudinal direction, and the amount of the variation changes depending on temperature, humidity and the preservation state of tapes. Also, there are errors of relative positions among the magnetic heads on the head unit because of limitations of machining precision. Such width variations of magnetic tapes and errors of the relative positions among the magnetic heads as shown above, it could be predicted, have large influences on the tracking precision against further enhancement of the density of data tracks 3 of magnetic tapes. This is an issue that could not be managed only by the tracking servo technology for controlling the position of the head unit in the widthwise direction of the magnetic tape. Therefore, for example, JP 2003-173508 A proposes a technique of reading jogging servo signals, which have been written in the data tracks of the magnetic tape, by chips of the individual heads and then driving jogging actuators for the individual head chips based on the read signals to thereby control the jogging of the head chips.
In addition, JP 2001-35046 A discloses a technique of, in a running state of a flexible tape such as a magnetic tape, detecting width of the tape contactlessly at high precision and then controlling the tension of the magnetic tape so that the detected tape width becomes a targeted tape width. However, this is a technique of performing the detection of tape width as well as the control of the tape tension in the running state in order to achieve stable running and uniform take-up of the magnetic tape. That is, the technique is other than to control the tape tension in consideration of influences exerted on the tracking precision by such variations in the magnetic tape width or errors of the relative positions among the individual magnetic heads as described above.
For magnetic tapes, as described above, there is a tendency that the data track width will decrease more and more from this time on under the trend toward higher recording densities. As a result of this, there has emerged a fear that correct reading of data may be unachievable because of significant increases in deviation of the positional relation between magnetic head and data track, i.e., differences between servo track—data track distance and servo head—recording/reproducing head distance in the tape width direction caused by not only widthwise swings of the tape during recording and reproducing operations as described above but also occurrence of widthwise size changes of the magnetic tape due to changes in temperature, humidity and the preservation state of the tape. This point is described in detail below.
In conventional recording/reproducing method for magnetic tapes, data recording is performed at a specified recording track width. Preferably, this recording track width is as smaller as possible because the number of tracks on the magnetic tape can be increased so that the recording capacity is increased. On the other hand, a lower limit value for the recording track width is determined naturally from considerations of the tracking servo precision, dimensional differences due to differences in expansion coefficient between head unit and magnetic tape caused by temperature and humidity variations, changes in tape width due to the leaving after taking-up, and the like. As the recording track width decreases beyond this lower-limit value, the reproducing head becomes more likely to be misaligned from the recording track, which causes data signal reading to become usable, leading to increases in error rate. In order to allow larger margins of track misalignment during data reproduction, normally, the recording track width is so designed as to be larger than the track width relative to the reading reproducing head. Therefore, there occurs no problem only if the relative position between recording track and reproducing head after tracking servo falls within the margin range.
Among the factors of the relative position shifts between recording track and reproducing head, the widthwise swing of the tape during the running can be managed with improvement of the performance of the tracking servo. However, if there occur the dimensional differences and tape width changes due to the leaving after taking-up, caused by differences in expansion coefficient between head unit and magnetic tape owing to temperature and humidity variations, that is, if the difference between a distance from a servo head to the data head located farthest from the servo head (e.g., servo head 12a and data head 11b, or servo head 12b and data head 11a shown in FIG. 2) and its corresponding distance from the servo track and the data track becomes a significant value relative to the aforementioned margin, those differences would matter irrespectively of the performance of the tracking servo.
For instance, if the above-mentioned distance from a servo head to the data head located farthest from the servo head is 2500 μm, the recording head width is 20 μm and the track width of the reproducing head is 12 μm, then the difference margin of the reproducing head is only 4 μm or so on one side relative to the reference position. With respect to expansions of the head unit and the magnetic tape due to temperature and humidity, the difference due to humidity is larger, and therefore from the viewpoint of humidity expansion coefficient, in the case of a magnetic tape having a humidity expansion coefficient of 2×10−5/% RH, a humidity change of 40% RH during a reproduction relative to a recording would cause a difference of 2.0 μm at a maximum in this system. On this account, whereas data tracks in the vicinity of the servo track bear no problem with amplitudes of up to about 8 μm against in-running widthwise swings of the tape, the data track located farthest from the servo track undergoes a decrease of amplitude allowance to about 4 μm. This would considerably matter in further enhancement of the recording density of magnetic tapes in the future.
As a solution to such problems, the method described in JP 2003-173508 A has been proposed. However, in this method, since the jogging servo signals are written in the data tracks of the magnetic tape, data recording area inevitably decreases correspondingly. This would form an obstacle to implementing magnetic tapes of high recording density.